1 Field of the Invention
The present invention relates to the field of measuring the concentration of individual components of a multi-component system and more particularly to the use of gaseous absorption spectroscopy to measure gaseous component vapor densities in samples that may contain aerosols and that may be supersaturated in the sampled component or other gases.
2 Description of the Prior Art
Multi-component systems may be composed of air plus other gaseous components, such as carbon dioxide, methane or water vapor, for example. Since 1912, when F. E. Fowle described a spectroscopic means of determining the ambient concentration of aqueous vapor, efforts have been made to improve upon the accuracy of hygrometers, which measure the concentration of the component water vapor in air. Such efforts have also attempted to extend the effective operating life of hygrometers independently of conditions encountered in the field.
An ideal instrument for measuring the concentration of a component of a multi-component system must be able to withstand harsh environmental conditions without loss of functional efficiency, permit maintenance without disassembly of the instrument, have long intervals between required maintenance, have low operational power requirements, and have an electromagnetic radiation sensing element that has stable and repeatable response characteristics. Despite the efforts since Fowle's work to develop such instruments in the form of hygrometers, until the present invention, such instruments having all of the above ideal characteristics have not been developed.
Photoconductive infrared sensors have been used in the past for measuring vapor density by gaseous absorption spectroscopy. However, the response of photoconductors to incident radiation is inherently non-linear. Since saturation of the sensor occurs as the intensity of the radiation increases, as such saturation occurs the photoconductor output levels off even though increased radiation intensity is directed onto the photoconductor.
The conductivity of a photoconductive sensor is increased by heating and by incident radiation. To reliably relate variable conductivity of the photoconductive sensor to variable conditions in the volume of the system being sampled at any instant, the photoconductor and its optically viewed background must be maintained at a constant temperature. In existing designs that maintain the entire instrument at a constant temperature, such temperature control requires about 200 watts of operating power and requires a warm up period of about 30 minutes. The non-linearity characteristic and the temperature control requirements have substantially increased the difficulty encountered in attempting to design an ideal instrument for measuring the concentration of individual components of a multi-component system using photoconductive sensors.
In L. D. Nelson U.S. Pat. No. 4,394,575, filed June 30, 1980, issued July 19, 1983 for APPARATUS FOR MEASURING VAPOR DENSITY, GAS TEMPERATURE, AND SATURATION RATIO, a lead selenide photoconductor for measuring gas temperature was disclosed. In that Patent, mention was made that other sensors capable of detecting infrared radiation such as, for example, bolometers or photoconductors or photovoltaic cells of other materials could be substituted for the lead selenide photoconductor without changing the instrument or method of measuring gas temperature. Although reference was there made to photovoltaic cells for gas temperature measurements, in 1980 relatively little was known about the application of photovoltaic cells in optical instruments for measuring vapor density.
Apart from the limitations presented by photoconductive sensors, other efforts have been directed to improving the structure of hygrometers. For example, an infrared source has been used to produce two identical beams which are alternately intercepted and directed through separate chambers. In such system, a first chamber through which the first beam is transmitted must be maintained in a dry condition to act as a reference. The other chamber through which the other beam is transmitted is provided with water vapor-bearing atmosphere to be measured. In such system, the difference in the intensities of the separate beams that are transmitted through the separate enclosures is used to indicate humidity in the ambient chamber. The accuracy of such system is dependent upon maintaining a constant dry condition in the reference chamber.
Attempts have been made to compensate for the non-linearity of photoconductors by using relatively complicated circuitry. One example is disclosed in A. Rekai U.S. Pat. No. 3,820,398 issued July 28, 1974 for "System for Providing a Linear Output from a Non-Linear Condition Responsive Device". The provision of such complex compensating circuitry is to be avoided in the ideal instrument for measuring the concentration of individual components of a multi-component system.